1. Field of the Invention
The present invention relates to systems with multiple power sources and, in particular, to power supplies having multiple power sources and a switching network used to selectively turn such power sources on and off.
2. Description of the Related Art
Various types of systems, such as power supplies, employ multiple power sources, which are typically used to drive a load in the system. For example, some power supply circuits have both AC and DC input voltage sources. These sources are typically selectively switched "on" or "off" with switches, to selectively couple or decouple, respectively, the power source to the load.
Referring now to FIG. 1, there is shown a circuit diagram of a prior art power supply circuit 100. Circuit 100 contains load R.sub.L1, which may be selectively driven by either AC or DC power. Load resistor R.sub.L1 is shown twice in FIG. 1, but may be physically the same load. To drive the load with AC, AC power source 101 is applied to AC to DC switching supply 102, which provides a DC output 103. DC power source 111 may also be applied to the load. A typical output for DC output 103 and DC power source 111 is -48 V with respect to ground.
Circuit 100 comprises AC connection network 121, DC connection network 123, and power source switching network 122. AC connection network 121 comprises load R.sub.L1, resistors R.sub.12 and R.sub.13, and transistor Q.sub.10. DC connection network 123 comprises load R.sub.L1 (which, as noted above, may be physically the same as load R.sub.L1 of network 121), resistors R.sub.14 -R.sub.17, and transistors Q.sub.11 and Q.sub.12. Switching network 122 comprises three switches: AC switch SW.sub.11, interlock switch SW.sub.12, and DC switch SW.sub.13. As illustrated, each switch (which has one or more "no contact" terminals labeled "NC" in FIG. 1) is shown in the "off" position, so that both AC and DC power sources are "off", or not coupled to the load R.sub.L1. The configuration illustrated may be referred to as the initial position. Interlock switch SW.sub.12 is a double-pole double-throw momentary switch, having dual switches (illustrated as coupled by a dashed line), which move together. AC and DC switches SW.sub.11 and SW.sub.13 are single-pole double-throw switches.
The switches of switching network 122 may be selectively switched to turn on the AC power source 101, to turn on the DC power source 111, or to turn both power sources on or off. There is typically a need to manually turn both AC and DC power sources on or off, which may be referred to as the manual switching function. There is also typically a need for an "interlock switch" capability or function, that ensures that neither power source is on, when the interlock switch is off, regardless of the state of any other switches in the switching network. Further, there is a need to isolate the switches, so that no current flows through the switches from the AC to the DC side of circuit 100, or vice-versa, regardless of the on or off state of the switches.
The interlock capability may typically be required where, for example, circuit 100 is implemented on a circuit board that is part of a power module, which module can reside in a housing or system carrier. Whenever the power module is physically outside of the housing, the interlock switch should be off, for safety and other reasons, to ensure that the system is not energized by either power source when the components are exposed. Thus, the interlock switch may be configured so that whenever the power module is securely seated within the housing, the interlock switch is on, and automatically off otherwise. Whenever the interlock switch is on, the user needs to have the ability to selectively switch the AC and DC power sources on or off. In addition, the switching network needs to be isolated in the sense described above.
In switching network 122, as described below, switch SW.sub.12 is the interlock switch, AC switch SW.sub.11 controls whether the AC power source 101 is on or off (when the interlock switch SW.sub.12 is on), and DC switch SW.sub.13 controls whether the DC power source 111 is on or off (when the interlock switch SW.sub.12 is on). When interlock switch SW.sub.12 is off, both AC and DC power sources are off, regardless of the state of switches SW.sub.11 and SW.sub.13.
To ensure that both power sources are turned off whenever the power module is not fully seated in the housing, interlock switch SW.sub.12 is adapted so that it is only on when pressure caused by filly seating the power module turns it on. Thus, when interlock switch SW.sub.12 is off, as illustrated in FIG. 1, node 131, at the junction of resistors R.sub.12 and R.sub.13 and the base terminal of transistor Q.sub.10, is grounded. Thus, transistor Q.sub.10 is turned off, and no current flows through load R.sub.L1 from the AC power source 101. In this case, AC connection network 121 does not serve to connect or couple AC power soure 101 to load R.sub.L1, since interlock switch SW.sub.12 being off causes AC connection network 121 to fail to allow such a connection.
Similarly, node 132, at the junction of resistors R.sub.14 and R.sub.15 and the base terminal of transistor Q.sub.11, is not coupled to ground, and a current thus flows through resistor R.sub.15, driven by DC power source 111, which gives rise to a voltage at node 132 sufficient to turn on transistor Q.sub.11. This diverts current flowing through resistor R.sub.17 through transistor Q.sub.11 instead of through resistor R.sub.16, and effectively grounds the base terminal of transistor Q.sub.12. Thus, transistor Q.sub.12 is turned off, and no current flows through load R.sub.L1 from the DC power source 111. In this case, DC connection network 123 does not serve to connect or couple DC power source 111 to load R.sub.L1, since interlock switch SW.sub.12 being off causes DC connection network 123 to fail to allow such a connection.
It can be seen that both power sources are turned off as long as interlock switch SW.sub.12 is off, since node 131 is still grounded, and node 132 is not grounded, regardless of the state of switches SW.sub.11, SW.sub.13. Whenever interlock switch SW.sub.12 is off, i.e. whenever the power module is not fully seated in the housing, circuit 100 may be said to be in interlock mode. Alternatively, when interlock switch SW.sub.12 is on, circuit 100 is not in interlock mode, and, therefore, AC switch SW.sub.11 can be used to turn the AC power source 101 on or off, and the DC switch SW.sub.13 can be used to turn the DC power source 111 on or off, as described below.
To turn on the AC power source 101, interlock switch SW.sub.12 must be turned on, and switch SW.sub.11 is also turned on, to activate AC connection network 121. Turning on AC switch SW.sub.11 when the circuit is not in interlock mode decouples node 131 from ground, thereby allowing transistor Q.sub.10 to turn on, which allows current to flow from the AC power source's DC output 103 through load R.sub.L1. Thus, when circuit 100 is not in interlock mode, turning on AC switch SW.sub.11 activates AC connection network 121, which causes AC power source 101 to be coupled to load R.sub.L1.
Similarly, to turn on the DC power source 111, interlock switch SW.sub.12 must again be turned on, and switch SW.sub.13 is also turned on. This couples node 132 to ground, thereby allowing transistor Q.sub.12 to turn on, which allows current to flow from the DC power source 111 through load R.sub.L1. Thus, when circuit 100 is not in interlock mode, turning on DC switch SW.sub.13 activates DC connection network 123, which causes DC power source 111 to be coupled to load R.sub.L1.
In typical usage, switches SW.sub.11 and SW.sub.13 are both turned on, so that both power sources are applied to load R.sub.L1. DC power source 111 may serve as a back-up in case of power failure or irregularities in AC power source 101. During normal operation, DC power source 111, which may be a rechargeable battery, may be constantly re-charged by DC output 103 from AC power source 101. Whenever AC power source 101 fails, for example due to a power outage, DC power source 111 drives load R.sub.L1 with no loss of continuity. Whenever the power module is removed from its housing, pressure is released from the back plane of the circuit board, which contains the pressure-sensitive portion of switch SW.sub.12, thereby causing interlock mode to be established and turning off both power sources.
Switching network 122 also performs the isolation function described above, since no switch configuration provides a current path between nodes 132 and 131. This is accomplished due in part to the fact that interlock switch SW.sub.12 is a double-pole double-throw switch, having two separate switch bars. Thus, switching network 122 is isolated, such that switching network 122 does not provide a current patch between AC connection network 121 and DC connection network 123.
Unfortunately, circuit 100 requires three separate switches in switching network 122 to perform the above-described functions. Each switch required in a circuit such as power supply circuit 100 adds extra expense and complexity (for example, to make switches other than the interlock switch accessible to users), and requires scarce printed-wiring board (PWB) area.